This invention relates to a silicon carbide (SiC) semiconductor device, more particularly to a silicon carbide semiconductor device forming a high resistant silicon carbide single crystal layer on a silicon single crystal substrate.
Semiconductor devices including diodes, transistors, integrated circuits (ICs), large scale integration (LSI) circuits, light emitting diodes, semiconductor lasers, charge coupled devices (CCDs) that are, made of silicon (Si) and compound semiconductor materials such as, gallium arsenide (GaAs) and gallium phosphide (GaP), are widely used in the fields of electronics. A silicon carbide semiconductor, meanwhile, has such features among others, as a wider forbidden energy gap (2.2 to 3.3 eV) than the semiconductor materials stated above, and thermal, chemical of mechanical stabilities and great resistance to radiation damage. Accordingly semiconductor devices employing silicon carbide can be used in severe conditions including high temperature, large amounts of electric power, exposure to radiation and other conditions where devices made of other semiconductor materials cannot be used. The silicon carbide devices are expected to be applied in a wide range of fields where semiconductor devices having a high amount of reliability and stability are necessary.
The reason why the silicon carbide semiconductor device has not yet been in practical use, despite expectation from various fields, is since the delay of the establishment of crystal growing techniques to obtain a high-quality and large-scale silicon carbide single crystal that are required in the mass production at an industrial scale where the productivity is important. Conventionally, diodes and transistors have been produced in a laboratory environment using silicon carbide single crystal grown by a sublimation-recrystallization method (called Lely method) or a silicon carbide single crystal layer epitaxially chemical vapor deposition process, a liquid phase epitaxy process or a similar process to the single crystal. This technique was reported by R. B. Campbell and H-C Chang as "Silicon Carbide Junction Devices," in "Semiconductors and Semimetals," eds. R. K. Williardson and A. C. Beer (Academic Press, N.Y., 1971) vol. 7 Past B. Chap 9 pp. 625 to 683. By this technique, however, only a small area of single crystal could be produced, and it was hard to control the dimension and shape of the single crystal area. Moreover, it was not easy to control a crystal polytype and the concentration of impurities existing in the silicon carbide crystal. Therefore, the technique for producing a semiconductor device by using silicon carbide was far short of the practical manufacturing method necessary at an industrial scale.
The present inventors have previously proposed a method for growing a high quality and large area of silicon carbide single crystal on a silicon single crystal substrate by a chemical vapor deposition method (CVD method). For example, this method is disclosed in U.S. Pat. No. 4,623,425, issued on Nov. 18, 1986, entitled "Method of Fabricating Single-Crystal Substrates of Silicon Carbide," by Akira Suzuki et al.
This method comprises the steps of forming a silicon carbide thin layer on a silicon single crystal substrate by a low-temperature CVD method and subsequently raising the temperature for growing silicon carbide single crystal by a CVD method. Thus a high quality silicon carbide single crystal layer having a large area in which the crystal polytype concentration of impurities, the dimensions, the shape and other factors controlled can be supplied by using a low-cost and easily obtainable silicon single crystal substrate. Furthermore, this manufacturing method is suitable for a mass production system and will provide high productivity. A semiconductor device using a silicon carbide single crystal layer produced on a silicon single crystal substrate in such a way has poor electrical insulation between the active device layer and the underlaying layer, so that preferable characteristics could not be obtained.